linux-kernel - Re: [PATCH] eCryptFS: fix missed mutex

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Message-Id: <20080520002832.89e74522.akpm@linux-foundation.org>
Date:	Tue, 20 May 2008 00:28:32 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Cyrill Gorcunov <gorcunov@...il.com>
Cc:	"Michael A. Halcrow" <mhalcrow@...ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] eCryptFS: fix missed mutex_unlock

On Sun, 18 May 2008 18:26:11 +0400 Cyrill Gorcunov <gorcunov@...il.com> wrote:

> ---
> 
> Ingo, could you please apply it and test? Actually I really doubt if it help
> with the locking problem you pointed. There are two procedures
> in miscrev.c - ecryptfs_miscdev_poll() and ecryptfs_miscdev_read()
> which takes/releases mutexes in a bit strange way... investigating,
> but this patch is needed anyway.
> 
> Index: linux-2.6.git/fs/ecryptfs/crypto.c
> ===================================================================
> --- linux-2.6.git.orig/fs/ecryptfs/crypto.c	2008-05-18 16:44:20.000000000 +0400
> +++ linux-2.6.git/fs/ecryptfs/crypto.c	2008-05-18 17:56:12.000000000 +0400
> @@ -1903,6 +1903,7 @@ int ecryptfs_get_tfm_and_mutex_for_ciphe
>  		if (rc) {
>  			printk(KERN_ERR "Error adding new key_tfm to list; "
>  					"rc = [%d]\n", rc);
> +			mutex_unlock(&key_tfm_list_mutex);
>  			goto out;
>  		}
>  	}

Better to do it this way, I think:

--- a/fs/ecryptfs/crypto.c~ecryptfs-fix-missed-mutex_unlock
+++ a/fs/ecryptfs/crypto.c
@@ -1906,9 +1906,9 @@ int ecryptfs_get_tfm_and_mutex_for_ciphe
 			goto out;
 		}
 	}
-	mutex_unlock(&key_tfm_list_mutex);
 	(*tfm) = key_tfm->key_tfm;
 	(*tfm_mutex) = &key_tfm->key_tfm_mutex;
 out:
+	mutex_unlock(&key_tfm_list_mutex);
 	return rc;
 }
_

Holding the lock for an additional few instructions may not be strictly
needed, but we might avoid the reintroduction of such bugs?

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